M474

The M474 is the front-end buffer amplifier used in the attenuator modules of the 11A32 and 11A34.

Regarding the M474, John Addis recalls

I started the design and Ivan “John” Cousins made a very important contribution to it, namely the addition of Q47 and Q48 and carried it into production. The IC is within the dashed lines. Outside the dashed lines are two JFETs (probably a matched pair of chips) and a (Tek-made?) fast PNP chip. This schematic is of a hybrid, showing the stray inductances. If I remember correctly, the M474 hybrid is separate from but next to the attenuator hybrid inside the attenuator assembly. The M474 likely tested and trimmed (resistors not shown here) for dc balance before being mounted next to the attenuator hybrid. R15 is likely trimmed when the M474 is in wafer form. R15, R1, R2, and circuitry create the 50 ohm output impedance to match the 11 inch 50 ohm Peltola cable from the attenuator to the M377 input.

The attenuator would have been trimmed for input C, attenuation, and capacitive compensation after being mounted and attached to the M474 tested amplifier.

The M474 contains bootstrap circuitry for the upper JFET so that its operating point does not change with input voltage. (the lower JFET operating point does not change with signal). This reduces the JFET capacitive loading on the attenuator, makes compensation independent of input signal and, more importantly keeps the dc linearity of the whole buffer amplifier good for the ±2V of possible input signal given the ±1V of dc offset applied to the M377 (11A32/11A34 input amplifier).

At the time I did not know how to make a Zener diode-like voltage source (that came in the M377), so there are embarrassing instances of multiple diodes in series.

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  Schematic of M474 and surrounding components

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  M474 and surrounding components designated by "X 0.95" and highlighted in red.